Mullite-containing investment casting core

ABSTRACT

An investment casting core includes a mullite-containing core body. The body can be made by providing a mullite-containing powder, forming the powder into a green body, and sintering the green body to form the mullite-containing core body.

BACKGROUND

This disclosure relates to investment casting and, more particularly, toinvestment casting cores.

Investment casting is known and used to cast metallic components withrelatively complex geometries. For example, gas turbine enginecomponents, such as airfoils that have internal passages, are made byinvestment casting. To form the internal passages or other relativelycomplex geometrical features, a core is provided which represents apositive projection of negative features that are to be formed in thecasting process. A wax material is then provided around the core in theshape of the component to be cast. A shell is then formed around the waxand the wax is removed to form a cavity between the core and thesurrounding shell into which molten metal is poured to form the shape ofthe component. After solidification of the metal, the core is removedusing known techniques to provide the cast component.

SUMMARY

An investment casting core according to an exemplary aspect of thepresent disclosure includes a mullite-containing core body.

In a further non-limiting embodiment of any of the foregoing examples,the mullite-containing core body has, by weight, 40% or greater ofmullite.

In a further non-limiting embodiment of any of the foregoing examples,the mullite-containing core body has, by weight, 40-90% of mullite.

In a further non-limiting embodiment of any of the foregoing examples,the mullite-containing core body has a material selected from the groupconsisting of alumina, silica, magnesia, yttria, calcia, zirconiumsilicate and combinations thereof.

In a further non-limiting embodiment of any of the foregoing examples,the mullite-containing core body has, by weight, 40-90% of mullite and60-5% of silica.

In a further non-limiting embodiment of any of the foregoing examples,the mullite-containing core body has a material selected from the groupconsisting of magnesia, yttria, calcia and combinations thereof.

In a further non-limiting embodiment of any of the foregoing examples,the mullite-containing core body has zirconium silicate.

A further non-limiting embodiment of any of the foregoing examplesincludes a metallic core body joined to the mullite-containing corebody.

In a further non-limiting embodiment of any of the foregoing examples,the metallic core body is selected from the group consisting ofmolybdenum, tungsten, tantalum, rhenium, niobium and combinationsthereof.

In a further non-limiting embodiment of any of the foregoing examples,the mullite-containing core body has, by weight, 80-90% mullite, 15-5%silica, 2.5-10% alumina, 0-2.5% zircon, 0-5% of one or more of: calcia,magnesia and yttria.

In a further non-limiting embodiment of any of the foregoing examples,the mullite-containing core body has a multi-modal or bi-modal grainsize distribution.

A method of making an investment casting core according to an exemplaryaspect of the present disclosure includes providing a mullite-containingpowder, forming the mullite-containing powder into a green body, andsintering the green body to form a mullite-containing core body.

A further non-limiting embodiment of any of the foregoing examplesincludes providing the mullite-containing powder in a blend with abinder, and the forming includes molding the blend into the green body.

In a further non-limiting embodiment of any of the foregoing examples,the mullite-containing powder has at least a bi-modal particle sizedistribution of mullite particles.

In a further non-limiting embodiment of any of the foregoing examples,the mullite-containing powder has a material selected from alumina,silica, magnesia, yttria, calcia, zirconium silicate and combinationsthereof in a powder with at least a bi-modal particle size distribution.

A further non-limiting embodiment of any of the foregoing examplesincludes joining the mullite-containing core body to a metallic corebody.

A further non-limiting embodiment of any of the foregoing examplesincludes selecting a composition of the mullite-containing powder toobtain a coefficient of thermal expansion in the mullite-containing corebody that matches a coefficient of thermal expansion of the metalliccore body.

A method of investment casting according to an exemplary aspect of thepresent disclosure includes casting a metallic component at leastpartially around an investment casting core that has amullite-containing core body.

A further non-limiting embodiment of any of the foregoing examplesincludes the mullite-containing core body that has a material selectedfrom the group consisting of alumina, silica, magnesia, yttria, calcia,zirconium silicate and combinations thereof.

A further non-limiting embodiment of any of the foregoing examplesincludes a metallic core body joined to the mullite-containing corebody.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features and advantages of the present disclosure willbecome apparent to those skilled in the art from the following detaileddescription. The drawings that accompany the detailed description can bebriefly described as follows.

FIG. 1 shows an example investment casting core that has amullite-containing core body.

FIG. 2 shows an example method of making an investment casting core andan example method of investment casting.

DETAILED DESCRIPTION

FIG. 1 schematically illustrates selected portions of an exampleinvestment casting core 20 (hereafter “core 20”). As can be appreciated,the core 20 is shown schematically for purposes of description and theactual geometry will depend on the design features of the castcomponent. As an example, the cast component can be an airfoil for a gasturbine engine, however, the examples herein are also applicable toother engine components or non-engine components.

The core 20 includes a mullite-containing core body 22. A “body” is amain or central foundational part, distinct from subordinate features,such as coatings or the like that are supported by the underlying bodyand depend primarily on the shape of the underlying body for their ownshape.

In the investment casting of metallic components, such as from asuperalloy material, the mechanical characteristics of a core influencethe quality of the investment casting process and component. As anexample, when molten metallic material is cast around a core,solidification of the metallic material exerts a force on the core. Ifthe Young's Modulus is to high, the core will resist the solidificationforces and may undesirably cause high internal stresses in the castcomponent or “hot tearing” of the component. In this regard, themullite-containing core body 22 has a composition that provides“crushability” of the core 20 during investment casting. That is, thecore 20 is tailored to “crush” under the solidification forces to reduceinternal stresses and hot tearing.

In one example, the mullite-containing core body 22 has a compositionthat has, by weight, 40% or greater of mullite. In a further example,the mullite-containing core body 22 has, by weight, 40-90% of mullite.

In further examples, additional materials are used with the mullite toadjust a coefficient of thermal expansion (“CTE”) of themullite-containing core body 22 and/or as processing aids. For example,the mullite-containing core body 22 has one or more materials selectedfrom: alumina, silica, magnesia, yttria, calcia, zirconium silicate orcombinations thereof. The alumina, silica, magnesia, yttria and calciamay be added to adjust the CTE. The yttria, calcia and zirconiumsilicate may be added as processing aids (yttria and calcia) tofacilitate sintering when making the mullite-containing core body 22 oras grain growth inhibitors (zirconium silicate). In one further example,the mullite-containing core body 22 has, by weight, 40-90% of mulliteand 60-5% silica. In further examples, the mullite-containing core bodycan also include up to 25% by weight of alumina, up to 5% by weight ofmagnesia, up to 5% by weight of yttria, up to 5% by weight of calcia andup to 10% by weight of zirconium silicate. Further, themullite-containing core body 22 can have a grain size that has a grainsize distribution that is equal to or approximately equal to a powdersize distribution, such as a bi-modal or multi-modal particle sizedistribution as discussed below. This, the grain size distribution canbe multi-modal or bi-modal.

The core 20 has a shape that corresponds to negative features that areto be formed in the investment cast component, such as gas turbineengine airfoils. In this regard, the illustrated core 20 has elongatedlimbs 24 and branches 26 that extend off of the limbs 24. It is to beunderstood, however, that the core 20 may alternatively have differentgeometries than shown, depending upon the features that are to be formedin the investment cast component.

In a further example, the core 20 can be a multi-material core that, inaddition to the mullite-containing core body 22, also has a metalliccore body 28 that is joined with the mullite-containing core body 22.For example, the metallic core body 28 is a refractory metal core. Inone example, the refractory metal is selected from molybdenum, tungsten,tantalum, rhenium, niobium and combinations thereof.

The metallic core body 28 can be joined to the mullite-containing corebody 22 using an adhesive material. For instance, the adhesive materialis applied the joining surface of the mullite-containing core body 22,the metallic core body 28 or both in the form of slurry, such as acolloidal silica-based slurry. The colloidal silica based adhesiveslurry will be comprised of materials of similar composition to the maincore body: mullite, alumina, zirconia silicate to act as filler materialas well as to ensure compatibility with the main body core. Upon dryingof the slurry, the colloidal silica serves as a binder adhesive tosecure the mullite-containing core body 22 and the metallic core body 28together. Alternatively, other types of adhesives and ceramic materialscould be used including adhesive systems based upon, ethyl silicate,sodium silicate, collidial alumina, colloidal yttria, colloidalzirconia.

The composition of the mullite-containing core body 22 can be tailoredwith regard to CTE to closely match the CTE of the metallic core body28. For example, for the given refractory metals described above,mullite provides a relatively close match in CTE. The mullitecomposition can be modified with the alumina, silica, magnesia orcombinations thereof to modify the CTE of the mullite-containing corebody 22 in accordance with the CTE of the selected refractory metal ofthe metallic core body 28. In one example, the CTE of themullite-containing core body 22 is within about 2% of the CTE of themetallic core body 28. The close match between the coefficients ofthermal expansion provides a more durable joint with improveddimensional stability and thus the ability to achieve tighter tolerancesand increased quality.

FIG. 2 illustrates an example method 30 of making the investment castingcore 20. The method 30 includes steps 32, 34 and 36. Step 32 includesproviding a mullite-containing powder, step 34 includes forming themullite-containing powder into a green body, and step 36 includessintering the green body to form the mullite-containing core body 22.

In a further example, step 32 includes providing the mullite-containingpowder in a blend with a binder and then in step 34 molding the blend toform the green body. For example, the binder can be a thermoplasticmaterial, a wax material or a wax material with viscosity modifiers,such as stearate. In one example, the blend includes approximately10-20% by weight of the binder material and a remainder of themullite-containing powder.

The composition of the mullite-containing powder can be tailoredaccording to the desired composition of the mullite-containing core body22. That is, the mullite-containing powder can include mullite powderand powders of any or all of the other above-described materials ofalumina, silica, magnesia, yttria, calcia and zirconium silicate.

In a further example, the mullite-containing powder has at least abi-modal particle size distribution. The bi-modal distribution providesa blend of coarse and fine particles that facilitate forming the greenbody with relatively large sections and also with relatively thinsections. That is, the multi-modal particle size distributionfacilitates filling a mold to form a relatively complex green bodyshape. Powders of the added materials in addition to the mullite canalso include a multi-modal particle size distribution for similarpurposes. Given this description, one of ordinary skill in the art willbe able to determine a bimodal or multi-modal distribution to meet theirparticular needs. As examples, multi-modal distributions are describedin “Fitting Bimodal Particle Size Distribution Curves” J. M. Dallavalleet. al., Ind. Eng. Chem., 1951, 43 (6), pp 1377-1380, incorporatedherein by reference, and Reed, James, Introduction to the principles ofceramic processing, New York, A Wiley-Interscience publication, 1995,also incorporated herein by reference.

As also shown in FIG. 2, a method 40 of investment casting can beimplemented after forming the mullite-containing core body 22. Themethod 40 includes casting a metallic component at least partiallyaround the investment casting core 20 that has the mullite-containingcore body 22. The details of investment casting, aside from the core 20and mullite-containing core body 22, are generally known and thereforewill not be described in further detail herein. As can also beappreciated, the methods 30 and 40 need not be conducted together. Thatis, the methods 30 and 40 can be mutually independent.

Although a combination of features is shown in the illustrated examples,not all of them need to be combined to realize the benefits of variousembodiments of this disclosure. In other words, a system designedaccording to an embodiment of this disclosure will not necessarilyinclude all of the features shown in any one of the Figures or all ofthe portions schematically shown in the Figures. Moreover, selectedfeatures of one example embodiment may be combined with selectedfeatures of other example embodiments.

The preceding description is exemplary rather than limiting in nature.Variations and modifications to the disclosed examples may becomeapparent to those skilled in the art that do not necessarily depart fromthe essence of this disclosure. The scope of legal protection given tothis disclosure can only be determined by studying the following claims.

1. An investment casting core comprising: a mullite-containing corebody; and a metallic core body joined to the mullite-containing corebody, and the mullite-containing core body has a coefficient of thermalexpansion that is within 2% of a coefficient of thermal expansion of themetallic core body.
 2. The investment casting core as recited in claim1, wherein the mullite-containing core body has, by weight, 40% orgreater of mullite.
 3. The investment casting core as recited in claim1, wherein the mullite-containing core body has, by weight, 40-90% ofmullite.
 4. The investment casting core as recited in claim 1, whereinthe mullite-containing core body has a material selected from the groupconsisting of alumina, silica, magnesia, yttria, calcia, zirconiumsilicate and combinations thereof.
 5. The investment casting core asrecited in claim 1, wherein the mullite-containing core body has, byweight, 40-90% of mullite and 60-5% of silica.
 6. The investment castingcore as recited in claim 1, wherein the mullite-containing core body hasa material selected from the group consisting of magnesia, yttria,calcia and combinations thereof.
 7. The investment casting core asrecited in claim 1, wherein the mullite-containing core body haszirconium silicate.
 8. (canceled)
 9. The investment casting core asrecited in claim 1, wherein the metallic core body is selected from thegroup consisting of molybdenum, tungsten, tantalum, rhenium, niobium andcombinations thereof.
 10. The investment casting core as recited inclaim 1, wherein the mullite-containing core body has, by weight, 80-90%mullite, 15-5% silica, 2.5-10% alumina, 0-2.5% zircon, 0-5% of one ormore of: calcia, magnesia and yttria.
 11. The investment casting core asrecited in claim 1, wherein the mullite-containing core body has amulti-modal or bi-modal grain size distribution.
 12. A method of makingan investment casting core, the method comprising: providing amullite-containing powder; forming the mullite-containing powder into agreen body; sintering the green body to form a mullite-containing corebody; and joining the mullite-containing core body to a metallic corebody, and selecting a composition of the mullite-containing powder toobtain a coefficient of thermal expansion of the mullite-containing corebody that matches a coefficient of thermal expansion of the metalliccore body.
 13. The method as recited in claim 12, including providingthe mullite-containing powder in a blend with a binder, and the formingincludes molding the blend into the green body.
 14. The method asrecited in claim 12, wherein the mullite-containing powder has at leasta bi-modal particle size distribution of mullite particles.
 15. Themethod as recited in claim 12, wherein the mullite-containing powder hasa material selected from alumina, silica, magnesia, yttria, calcia,zirconium silicate and combinations thereof in a powder with at least abi-modal particle size distribution.
 16. (canceled)
 17. (canceled)
 18. Amethod of investment casting, the method comprising: casting a metalliccomponent at least partially around an investment casting core that hasa mullite-containing core body and a metallic core body joined to themullite-containing core body. the mullite-containing core body having acoefficient of thermal expansion that is within 2% of a coefficient ofthermal expansion of the metallic core body.
 19. The method as recitedin claim 18, wherein the mullite-containing core body has a materialselected from the group consisting of alumina, silica, magnesia, yttria,calcia, zirconium silicate and combinations thereof.
 20. The method asrecited in claim 18, further comprising a metallic core body joined tothe mullite-containing core body.
 21. The investment casting core asrecited in claim 1, wherein the mullite-containing core body includesyttria.
 22. The investment casting core as recited in claim 1, whereinthe mullite-containing core body includes calcia.
 23. The method asrecited in claim 18, wherein the mullite-containing core body includesyttria.
 24. The method as recited in claim 18, wherein themullite-containing core body includes calcia.
 25. The method as recitedin claim 12, wherein the mullite-containing core body includes yttria.26. The method as recited in claim 12, wherein the mullite-containingcore body includes calcia.
 27. The investment casting core as recited inclaim 1, wherein the metallic core body is adhesively joined to themullite-containing core body.